Cutting elements or inserts for use in rotary drill bits adapted to bore holes in rock are conventionally made entirely of a sintered mixture of tungsten carbide with about 15 to 17 percent cobalt. Such cutting elements are tough and fracture resistant (since fracturing of the cutting elements during the drilling process can not be tolerated) but are not as wear resistant as is desired. It is known that a sintered mixture of tungsten carbide and about 9 to 11 percent cobalt has significantly greater wear resistance than that containing cobalt in the 15 to 17 percent range, however, such wear resistant tungsten carbide is too prone to fracture to be used to form the entire cutting element. Thus, as is described in U.S. Pat. No. 4,359,335, attempts have been made to attach wear pads of such wear resistant tungsten carbide on bodies of such tough tungsten carbide to provide the advantage of both in one cutting element. As described in U.S. Pat. No. 4,359,335, this has been done by first forming the wear pad by pressing a mixture of tungsten carbide with about 9 to 11 percent cobalt in a first die cavity at pressures of about fifteen tons per square inch, positioning that pressed, unsintered wear pad in a second die cavity, positioning a second mixture of tungsten carbide and about 15 to 16 percent cobalt in the second die over the pad, pressing the second mixture into the die at a pressure of about 15 tons per inch, and then sintering the combination to form the cutting element or insert.
Our experience with this method, however, has been that while it may adequately bond small wear pads on surfaces of tip portions of cutting elements that project from sockets in a rotary drill bit in which base portions of the cutting elements are received, the portions of the tougher tungsten carbide material around the pads will contact rock being cut or crushed and will wear away rapidly when compared to the wear pads so that support for the wear pads is lost and they break away.
When we have attempted to form tip portions for cutting elements that are completely or almost completely covered or crowned by the wear resistant tungsten carbide material using the method described in U.S. Pat. No. 4,359,335, voids have been formed at the interface between the wear resistant crown and the underlying base portion of the tough tungsten carbide material during the sintering process, and the crown has had a strong tendency to crack off during use so that the cutting element is unacceptable.